State-Dependence of the Climate Sensitivity in Earth System Models of Intermediate Complexity

Pfister, Patrik; Stocker, Thomas (2017). State-Dependence of the Climate Sensitivity in Earth System Models of Intermediate Complexity. Geophysical Research Letters, 44(20), 10,643-10,653. American Geophysical Union 10.1002/2017GL075457

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Growing evidence from general circulation models (GCMs) indicates that the equilibrium
climate sensitivity (ECS) depends on the magnitude of forcing, which is commonly referred to as state-dependence. We present a comprehensive assessment of ECS state-dependence in Earth system models of intermediate complexity (EMICs) by analyzing millennial simulations with sustained 2×CO₂ and 4×CO₂ forcings. We compare different extrapolation methods and show that ECS is smaller in the higher-forcing scenario in 12 out of 15 EMICs, in contrast to the opposite behavior reported from GCMs. In one such EMIC, the Bern3D-LPX model, this state-dependence is mainly due to the weakening sea
ice-albedo feedback in the Southern Ocean, which depends on model configuration. Due to ocean-mixing adjustments, state-dependence is only detected hundreds of years after the abrupt forcing, highlighting the need for long model integrations. Adjustments to feedback parametrizations of EMICs may be necessary if GCM intercomparisons confirm an opposite state-dependence.

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